linux-kernel - Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests

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Date:   Tue, 4 Apr 2017 15:44:46 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Alexander Graf <agraf@...e.de>
Cc:     Jim Mattson <jmattson@...gle.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Gabriel L. Somlo" <gsomlo@...il.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        linux-doc@...r.kernel.org, qemu-devel@...gnu.org
Subject: Re: [PATCH v5 untested] kvm: better MWAIT emulation for guests

[Cc qemu-devel as we've gone off-topic]

2017-04-04 15:15+0200, Alexander Graf:
> On 04/04/2017 03:13 PM, Radim Krčmář wrote:
>> 2017-04-04 14:51+0200, Alexander Graf:
>> > Please see my patch to force enable CPUID bits ;).
>> Nice.  MWAIT could also use setting of arbitrary values for its leaf,
>> but a generic interface for that would probably look clunky on the
>> command line ...
> 
> 
> I think we should have an interface similar to smbios for that eventually.
> Something where you can explicitly set arbitrary CPUID leaf information
> using leaf specific syntax. There are more leafs where it would make sense -
> cache topology for example.

Right, separating cpuid from -cpu makes it bearable, like

  -cpuid leaf=%x[,subleaf=%x][,eax=%x][,ebx=%x][,ecx=%x][,edx=%x]

And Having multiple interfaces for the same thing would result in some
corner case decisions ...
I think QEMU should check that feature flags specified flags specified
by -cpu are not cleared by -cpuid.
I'm not sure if setters like "|=" and "&=~" would be beneficial in some
cases.